1. Field of the Invention
A polycondensation reaction is a chemical reaction in which a macromolecule is built up stepwise (Emons, H. H.; Fedtke, M.; Hellmond, P.; Landschulz, G.; Poschl, R.; Pritzkow, W.; Ratzsch, M.; Zimmermann, G.; Lehrbuch der Technischen Chemie [Textbook of Industrial Chemistry]; VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1984). Each step in the condensation produces a reaction product which is in equilibrium with other reaction constituents. The reaction is therefore an equilibrium reaction. In each case the reaction takes place between two different functional groups of the starting materials (monomers), and at each stage of the reaction a low-molecular-weight substance (e.g. water, hydrogen halides, alcohols, etc.) is eliminated, with simultaneous lengthening of a polymer chain by one monomeric building block. Products of the reaction are therefore the macromolecule and the corresponding low-molecular-weight elimination products, in equilibrium with the starting materials.
If a high conversion is desired from a polycondensation reaction the low-molecular-weight elimination products must be removed from the equilibrium in order to shift the reaction equilibrium toward the products. If the monomers are dissolved in a solvent at the beginning of the reaction, there may also be a need for the solvent likewise to be removed from the reaction mixture. It is possible here to use the low-molecular-weight elimination products as a solvent.
When the low-molecular-weight elimination products and the solvent, if used, are removed the viscosity of the reaction mixture can change from a low-viscosity solution (e.g. similar to water) at the beginning of the reaction to give a high-viscosity polymer melt or polymer solution at the end of the reaction. Indeed, it is frequently necessary to remove the low-molecular-weight elimination products and the solvent, if used, through as far as a dry solid if the desired conversion in the reaction is to be achieved.
2. Description of the Related Art
The familiar method for removing the low-molecular-weight elimination products and the solvent, if used, is distillation. This means that while the polycondensation reaction progresses the low-molecular-weight elimination products and, respectively, the solvent, if used, are removed by evaporation, either simultaneously or in stages (alternating reaction and distillation).
Chemical reactors for carrying out polycondensation reactions therefore have two tasks. They must be able to mix and transport the reaction mixture efficiently at low, and also at high, viscosities (where appropriate through as far as dry solids) and at the same time allow removal by evaporation of the low-molecular-weight elimination products and/or also the solvent from the reaction mixture.
The following reactors are used in prior art methods for polycondensation reactions:
Screw Reactors
High-capacity screw reactors of ZDS-R type have been used by OCKER, Werner and Pfleiderer, Stuttgart since as early as 1962 for polycondensing polyesters. The devices are used at low rotation rates and with long residence times (from 1 to 2.5 hours). The process is described in Herrmann: Schneckenmaschinen in der Verfahrenstechnik [Screw Devices in Processing], Springer Verlag 1972. A disadvantage of these devices is their low mixing efficacy, due to the low rotation rates.
Disk Reactors (Zimmer, Frankfurt Am Main)
This type of reactor is a cost-effective alternative to the screw reactor and is nowadays used worldwide for polyester production. The principle on which the reactor is based is that of slowly rotating disks which produce melt films and thin layers which form a large surface for the transfer of material. In the usual embodiment, the disk reactors are not self-cleaning. One version of the reactor which has been equipped with strippers to improve self-cleaning is still being tested on a pilot scale. Like the screw reactor, the reactor can be used over a wide viscosity range. However, its functioning requires that the melt be capable of forming a reservoir. Conversion to a non-flowable paste or to the solid is not possible.
Twin-screw Extruders
Recently, corotating twin-screw extruders with low capacity and high rotation rates have been used for polycondensation. Example: ZSK type from Werner and Pfleiderer, Stuttgart or ZE type from Berstorff, Hanover. GREVENSTEIN, A.: Reaktive Extrusion und Aufbereitung [Reaction Extrusion and Product Treatment], Carl Hanser Verlag 1996, gives polyethylene terephthalate (PET), polybutylene terephthalate (PBT), copolyesters, polyimide (PI) and polyetherimide (PEI) as applications. The efficacy of mixing is good due to the high rotation rates. At the same time there is high shear and dissipation of energy, and this can have an adverse effect on product quality of sensitive polymers. However, the low capacity of the reactor means that this type is of interest only for processes which require a low residence time (generally &lt;1 minute). For this reason industrial use is mostly restricted to postcondensation.
Grid-cage Reactors (e.g. Werner and Pfleiderer)
This type of reactor supplies a large reaction capacity and therefore long residence times, and it is used on an industrial scale for polycondensation reactions. However, compared with the other types it has restrictions with regard to the maximum polymer viscosity which can be processed.
High-capacity Kneading Reactors (e.g. List)
This type approaches the twin-screw extruder in its mixing efficiency and kneading efficiency. However, large capacity means that it is also possible to realize high residence times. Unlike reactor types 1 to 3, however, the axial back mixing and transporting action of these reactors is highly viscosity-dependent, i.e. at low to moderate viscosity back-mixing is at a high level and transporting action is poor. This type of reactor is therefore of relatively little interest industrially for use with low-viscosity media.